Tachyarrhythmias are abnormal heart rhythms characterized by a rapid heart rate. Tachyarrhythmias generally include supraventricular tachyarrhythmia (SVT, including atrial tachyarrhythmia, AT) and ventricular tachyarrhythmia (VT). Fibrillation is a form of tachyarrhythmia further characterized by an irregular heart rhythm. In a normal heart, the sinoatrial node, the heart's predominant natural pacemaker, generates electrical impulses, called action potentials, that propagate through an electrical conduction system to the atria and then to the ventricles of the heart to excite the myocardial tissues. The atria and ventricles contract in the normal atrio-ventricular sequence and synchrony to result in efficient blood-pumping functions indicated by a normal hemodynamic performance. VT occurs when the electrical impulses propagate along a pathologically formed self-sustaining conductive loop within the ventricles or when a natural pacemaker in a ventricle usurps control of the heart rate from the sinoatrial node. When the atria and the ventricles become dissociated during VT, the ventricles may contract before they are properly filled with blood, resulting in diminished blood flow throughout the body. This condition becomes life-threatening when the brain is deprived of sufficient oxygen supply. Ventricular fibrillation (VF), in particular, stops blood flow within seconds and, if not timely and effectively treated, causes immediate death. In very few instances a heart recovers from VF without treatment.
Cardioversion and defibrillation are used to terminate most tachyarrhythmias, including AT, VT, and VF. An implantable cardioverter/defibrillator (ICD) is a cardiac rhythm management (CRM) device that delivers a cardioversion/defibrillation pulse (also known as a shock) to terminate a detected tachyarrhythmia episode by depolarizing the entire myocardium simultaneously and rendering it refractory.
Because the cardioversion/defibrillation pulse causes pain in the patient and drains a substantial amount of energy from the ICD's battery, its delivery is justified when it is necessary, such as when a life-threatening VT or VF is occurring. An unnecessary delivery of the cardioversion/defibrillation pulse causes pain and shortens the longevity of the ICD without benefiting the patient. A detection of tachyarrhythmia based on heart rate is not sufficient for determining the need and adequacy of delivering the cardioversion/defibrillation pulse. For example, a transient, non-sustaining VT may terminate itself without device intervention.
For these and other reasons, there is a need for determining whether to deliver the cardioversion/defibrillation pulse based on the nature of a detected tachyarrhythmia episode.